1. Technical Field
The present invention generally relates to a system and method for measuring dimensions of a specimen and, more particularly, to a scanning electron microscope (SEM) for measuring dimensions of a specimen such as a semiconductor wafer.
2. Description of the Related Art
Scanning electron microscopes are widely used to observe the configurations of various types of specimens. In such microscopes, an electron beam is scanned over the surface of the specimen. The electron beam causes electrons on the surface of the specimen to be ejected. These "secondary" electrons are detected and used to generate a picture of the surface either on a screen or in a photograph. Measurements may be determined from the picture and the scale factor of the scanning electron microscope.
To obtain high resolution images of a specimen, the electron beam of the scanning electron microscope must be properly focused. The scanning electron microscope may include a so-called automatic focus (autofocus) feature for obtaining such proper focus. U.S. Pat. No. 5,512,747 to Maeda describes scanning electron microscopes having autofocus features. Referring to FIG. 1(a), an electron beam 1 generated from an electron gun (not shown) is scanned by deflection coils 2X and 2Y and focused into a narrow beam which is irradiated onto a specimen 4. Specimen 4, for example, may be a semiconductor wafer on which semiconductor circuit elements have been or are being formed. Secondary electrons generated from the specimen 4 are detected by a detector 5. By using an electron signal amplified by an amplifier 6 as a brilliance modulation signal for a monitor (not shown) and by synchronizing the signal with scanning by the deflection coils 2X and 2Y, a scanning image of the specimen 4 may be provided on the monitor screen. Stage 15 uses coordinate data from a wafer information register 16 to move the wafer to various measuring points. A focus condition detector 10 uses the absolute value of time differential (or position differential) of the electron signal detected by the detector 5 as an index for evaluating the focus condition. For performing focusing, the exciting current of an objective lens 3 is changed sequentially and gradually by a focus controller 7. Each exciting current makes the electron beam scan on the specimen 4. The intensity of the secondary electron signal obtained at each exciting current is integrated and the absolute value of the signal for a certain scanning period is differentiated by a signal intensity integrator 8. As a result of the above calculation, a focus evaluation value corresponding to each focus condition is obtained as an output from signal intensity integrator 8. It is assumed that the beam is exactly focused when the focus evaluation value reaches a peak value. Accordingly, peak detector 9 detects the peak of the focus evaluation value and an exciting current that makes the focus evaluation value become the peak is sent to the objective lens 3 from focus controller 7, thereby performing focusing. This is also may be referred to as SEM autofocus with an electron beam.
FIG. 1(b) illustrates an arrangement that utilizes a picture processing technique for autofocus. This is an alternative to the SEM autofocus with the electron beam. Using the secondary electron signal from the amplifier 6 as a brilliance modulation signal of a monitor 21, which is scanned in synchronization with the deflection coils 2X and 2Y, an image of the specimen 4 is displayed on the monitor 21. Because the contrast of the picture becomes more intense as the electron beam 1 is focused on the specimen 4 more exactly, the picture signal serves as an index for evaluating the focus when the signals of adjacent picture elements are integrated or differentiated and the sum of their absolute values is calculated. Focus condition detector 10 uses this sum as a focus evaluation value. Picture processor 8 receives the picture signal from the monitor 21 and calculates the focus evaluation value. The peak detector 9 detects the peak of the focus evaluation value and an exciting current that makes the focus evaluation value become the peak is sent to the objective lens 3 from the focus controller 7, thereby performing focusing.
Other variations of SEM autofocus include focusing using optical hardware or static capacity sensors.
In practice, accurate autofocus is difficult to achieve. For example, optical autofocus systems such as the system of FIG. 1(b) are relatively accurate for large patterns, but tend to provide insufficient resolution for highly integrated patterns such as those associated with 256 Mbit DRAMs, for example. Electron beam autofocus systems such as the system of FIG. 1(a) provide better resolution than optical autofocus systems, but are subject to noise. A cause of this noise may include beam charging systems. As described in Chain et al., "Automated CD Measurement with the Hitachi S-6280", SPIE Vol. 2439, pages 319-324, the reliability of automated measurement sequences for measuring critical dimensions (CD) in VLSI manufacturing processes is dependent on various factors including proper focusing. Thus, if an image is out-of-focus due to a failure to accurately autofocus, the measured values of the dimensions of the features of the specimen will differ from the actual value. Further, if the autofocus system fails to operate properly, automated measurements (feature length or width, for example) may be impossible to perform.
Accordingly, it would desirable to provide a system and method for measuring the dimensions of the features of specimens in which the effects of a failure to properly autofocus a scanning electron microscope are eliminated.